1. Field of the Invention
The present invention relates to a semiconductor memory device and a method for producing the same.
2. Description of the Related Art
In the past, a semiconductor memory device in which a dynamic random access memory (1 megabit DRAM) and a gate array are formed on one silicon substrate has been known (CICC Resumes, Mar. 20, 1988). In this semiconductor memory device, as shown in FIG. 2, N-wells 22a, 22b and P-wells 23 are formed in an N-type silicon substrate 21. A DRAM 30 is formed inside and on the N-wells 22a, 22b. A cut P-well 31 or a gate array 32 is formed inside and on the P-wells 23. In the DRAM 30, a charge storage electrode layer 26a is formed by etching a first polysilicon layer to a predetermined pattern, and a transfer gate electrode layer 26b is formed by etching a second polysilicon layer to a predetermined pattern. In FIG. 2, the reference numeral 24 denotes a small amount of carriers, reference numeral 25 denotes a capacitor, reference numeral 27 denotes a first wiring layer made of Al, 28 a second wiring layer made of Al, and reference numeral 29 denotes a memory element.
However, according to the above construction, in which the charge storage electrode layer 26a is formed from the first polysilicon layer, and after that the transfer gate electrode layer 26b is formed from the second polysilicon layer, resulting in a formation of a DRAM, when a DRAM, a read only memory (ROM), and a driver are formed in combination on a substrate, a production process becomes complicated and moreover, each memory element occupies a wide area on the substrate, causing poor integration.